Thanks to the Curiosity rover's explorations, we now have even more evidence that there was once flowing water on the surface of the moon. Curiosity recent photographed what appears to be a dried stream bank:

Visible in the above image, for example, is a small overhanging rock ledge that was quite possibly created by water erosion beneath. The texture of the ledge appears to be a sedimentary conglomerate, the dried remains of many smaller rocks stuck together. Beneath the ledge are numerous small pebbles, possibly made smooth by tumbling in and around the once-flowing stream. Pebbles in the streambed likely fell there as the bank eroded. Circled at the upper right is a larger rock possibly also made smooth by stream erosion.